Try pushing in the plastic key pin again. IT takes alot of pressure to go between the spring loaded key capture. It will make a loud clunk when you get into the locked position. Then it take a rather hard pull to get back out again. All of this is necessary to keep from accidentally pulling the plastic key pin out.
Now that you know you are draining your good batteries leave the negative cable disconnected until you get all of this figured out... You don't want to further kill your brand new 6VDC batteries...
Well now we all know a little more of the mystery which is good hehe...
Roy Ken